by thomas » Sun Jun 09, 2019 10:10 am
Some random thoughts on the subject, though I'm probably missing something obvious. We don't though have much info to go on. I'm not sure what you mean by "Sometimes if you hold the ignition key on past its normal position it will attempt to fire up but not always." Do you mean hold the starter motor in operation after it would seem to have started? One possibility stemming from that is that there is some deficiency in the supply to the coil for normal running resulting possibly from a poor or intermittent connection in the circuit made to the coil through the ignition switch itself.
Almost always it's Fuel Or Spark, or both, or something else entirely.
Fuel: possibly weak fuel-pump, blocked fuel filter? The only effect of pumping the throttle is to squirt in the small reserve held in the accelerator pump if a down-draft Solex/Zenith; if as I think is the case here, it's a Stromberg CD150 I'd guess the effect of pumping the throttle is exactly nil, there won't be enough vacuum at cranking speeds for the carb-piston and needle to lift to admit more air and more fuel, however much the throttle-flap downstream was waggled, it might help only to clear excess fuel as you might do by holding the throttle open wide if flooded. So I'd hesitate to claim pumping the throttle is having no effect, but whether good or bad, it's probably negligible.
On the electrical side check check that the electronic ignition wires inside the distributor body haven't chafed against any rotating parts such as the the trigger rotor or moving parts of the centrifugal or vacuum advance mechanisms. Check that the resistance bypass wire from from the solenoid to the coil is supplying the required higher voltage when cranking over than the voltage the resistance wire feed is providing, and that the coil is intended for use with ballasted ignition system if that is what you have. It might be that your battery (or starter motor) is sub-optimal and the system voltage drop that results is too great when the starter is operating for the coil or electronic ignition to work well.
On my HB (on which the ballast was an external resistor sat atop the coil) when fitting the electronic ignition kit some years ago I removed the ballast resistor, replaced the ballasted coil with a coil intended for continous 12V operation, leaving the solenoid to coil (the bypass) wire redundant and thus disconnected it too at the solenoid end.
On the coldest possible morning some winters ago after the car had sat for a few days under the snow, I went out at the crack of dawn to start it and expected no issues, but though it spun over lustily it wouldn't fire. In the interests of science, I disconnected the positive feed from the loom to the coil and connected a bike battery's live to the coil's live terminal and bike battery earth to the engine block, cranked it over and it fired immediately first go. Conclusion: insufficient coil voltage when cranking.
As the battery declines from near new, with the starter drawing a heavier current for whatever reason, and cooler temperatures, thick oil etc., the ballasted sytem makes sense. I've been thinking (when I get the car back from its present protracted bout of bodywork) of putting the ballast system (coil, resistor and bypass wire) back.
With the HB at least, with its external ballast resistor, it is possible to connect the electronic ignition unit to the full 12-14v always present at the coil, while still running the lower voltage type of coil through the ballast resistor for normal running, just by how live feed, bypass feed, coil and resistor wiring is arranged atop the coil, so retaining the electronic ignition (which always gets the voltage its manufacturer recommended i.e. full) and still having the enhanced cold weather starting, through the bypass wire, of running the lower-voltage coil at the highest voltage the system can provide when cranking.